The invention pertains generally to a control system for an electrohydraulic actuator and is more particularly directed to a control system for an electrohydraulic actuator of the doser type.
The concept of a doser type electrohydraulic actuator is known in the art. These actuators are based on the fact that if a measured quantity or "dose" of hydraulic fluid is injected or exhausted from the control chamber of a differential area piston actuator, its output makes a step movement commensurate with the size of the dose. The dose can be administered periodically in an on-off fashion to achieve a stepping motor type response to the digitally administered doses.
It is further known that the dose volume can be controlled by opening a solenoid valve for a discrete time period in response to an electrical pulse of predetermined duration from an electronic controller. The effective output travel rate of the doser actuator can thus be varied by changing the pulse frequency and/or the pulse width with the maximum slew rate of the device limited by the flow capacity of the solenoid valve if held continuously open. When operated in this manner, these actuators are compatible with, and easily controlled by modern digital electronic controls to produce a stepper motor-like response of variable speed.
A doser-type electrohydraulic actuator is more fully described in a U.S. Pat. No. 4,256,017 entitled "Differential Area Electrohydraulic Doser Actuator", filed Apr. 5, 1979, in the name of James M. Eastman, which is commonly assigned with the present application. The disclosure of Eastman is hereby incorporated by reference herein.
However, unlike conventional stepper motors, doser actuators do not have inherent digital precision. This is so because, instead of dividing up the stroke of the actuator into precise small fractions for the steps, each dose is independently metered so the error is cumulative and there can be no precise correlation between the number of steps and output positions. Since, for most control applications, the positioning of an actuator is controlled in a closed loop manner based on the position of the controlled device, the available precision of a true stepping motor exceeds that which is necessary and doser type actuators can serve quite well in this capacity. The doser actuator will displace the stepper motor in many instances because of its lower cost and complexity while having a higher reliability.
An example of an adaptive closed loop control system for an electrohydraulic actuator is illustrated by U.S. Pat. No. 4,007,361 issued to Martin on Feb. 8, 1977. These closed loop actuator systems can be used to position various components of turbine engines such as fuel control valves, exhaust nozzles, and variable geometry vanes. Additionally, other aircraft uses may include the accurate positional control of the rudder, elevator, flaps or other components in response to pilot-initiated or automatic control systems inputs.
The equilibrium condition for closed loop operation of a doser actuator requires either a dead band for which no position correction is made until the positional error exceeds the effect of one minimum dose or step, or steady state limit cycling where the actuator takes a step, passes the desired position then steps backward past it, steps forward again, etc. A dead band is preferred because limit cycling reduces solenoid valve life and may detrimentally effect regulation of the parameter controlled by the actuator movement. However, for either equilibrium condition, steady state precision depends on having a small enough minimum doser step to accurately move the actuator piston to its final position. Smaller steps require shorter solenoid minimum on periods.
While it is true that the size of the dose can be made smaller with progressively shorter solenoid energization periods, it is equally true that if the dose is reduced substantially, its magnitude becomes more sensitive to second order effects. Thus, at small pulse widths not only is the actuator response nonlinear, but whether the pulse effects a change in actuator position at all becomes more uncertain. This is because each doser actuator has a threshold pulse width below which no actuator movement results. The magnitude of this threshold can vary significantly from unit to unit. Moreover, the threshold pulse width for each individual unit is load sensitive and depends upon the back pressure or pull on the actuation piston of the doser actuator at the time of pulse application. A pulse barely long enough to elicit an actuator response for an opposing load condition may produce a movement manyfold larger than the error tolerance range for an assisting load condition. This prohibits a steady state positioning of the actuator piston within a very narrow tolerance band.
However, there are conventional servo positioning actuators which employ torque motor operated valves. Although the torque motors are expensive, complex, and less reliable than the doser actuator, they act continuously down to very small error values. Therefore, to be competitive with these linear servo actuators, a doser actuator must be able to solve the problem of accuracy with respect to the threshold pulse width. To accomplish this, the control system must be able to supply pulse widths that are only marginally larger than the variable threshold pulses as equilibrium is approached.